Tibetan mothers have provided anthropologists with a prime example of ongoing human evolution. Researchers have found that women who are able to store more oxygen in their blood have more offspring that live to maturity.

Cynthia Beall, a physical anthropologist at Case Western Reserve University in Cleveland, Ohio, and colleagues travelled to the Himalayas to see if they could catch the population there in the act of adapting to the low levels of oxygen found at 4,000 metres.

Beall and her team lived in a series of villages, interviewing thousands of inhabitants, creating detailed family trees and, for women between 20 and 60, recording pregnancy histories.

They also estimated the concentration of oxygen in the villagers' blood, by shining a light through their fingertips. Haemoglobin in the blood absorbs different amounts of the light, depending on how saturated it is with oxygen.

Once non-genetic factors such as age, illness, or smoking were removed, a subset of the group seemed to have a blood-oxygen concentration that was 10% higher than normal. This trait was inherited in a way that suggested the difference was due to a single gene.

The researchers also found that the children of women with this putative gene are much more likely to survive to the age of 15, when they are old enough to have children of their own. In the low-oxygen group, each woman had on average 2.5 children that died during childhood. In the high-oxygen group, that average was just 0.4.

The result strongly suggests that the high-oxygen gene confers a reproductive advantage, and is spreading through the population, says Beall. The work will be published in the Proceedings of the National Academy of Sciences1.

Save the children

Beall acknowledges that the idea still has its gaps. It is unclear how a gene might increase blood oxygen levels, or how high oxygen levels in the mother might help her children survive. Maybe the trait allows the children to reach a higher birth weight or, if inherited, it might help children to survive bouts of respiratory illness that would otherwise prove fatal.

But if Beall is correct, this is a tidy example of darwinian evolution occurring as we watch. With so many more high-oxygen babies surviving, after 2,000 years the whole of the Himalayan population might sport this gene, Beall suggests.

She now hopes to learn more about why the low-oxygen babies die, partly to refine her theory and partly to help them. "It will be a wonderful opportunity to stop infant mortality," she says. Of course, if she succeeds, and survival rates in the two groups become similar, the population will promptly stop evolving.